The present invention relates to a method for producing a hybrid integrated circuit and, more particularly, to a method for producing a hybrid integrated circuit, including an improved step of mounting electronic parts on an insulating substrate.
A certain type of high-frequency electronic circuit such as a high-frequency amplifier cannot be realized in a monolithic integrated circuit by current semiconductor techniques and is therefore generally formed into a hybrid integrated circuit. Such a hybrid integrated circuit is assembled by mechanically mounting a plurality of discrete parts on a substrate. For this reason, hybrid integrated circuits are subject to large variations in characteristics and improvements in this respect have been desired. Furthermore, in a high-frequency integrated circuit, the mounting precision of electronic parts must be high.
Electronic parts such as capacitors or resistors are fixed with solder on a conductor formed on an insulating substrate. In this case, in order to improve the mounting precision of parts, the electronic parts are temporarily fixed on the substrate conductor with an adhesive containing mainly an epoxy resin or silicone resin and are thereafter fixed with solder. However, such an adhesive, once hardened, cannot be dissolved in a solvent. As a result, the hardened adhesive remains between the substrate and the electronic parts of the obtained hybrid integrated circuit. Therefore, the adhesive forces of the adhesive and solder act on a single electronic part in different magnitudes and at different points. Such forces generate stress at each adhered portion. This stress in turn results in a mechanical distortion and degrades the electrical characteristics of the circuit. In a worst case, the electronic part may be damaged in a temperature cycle test.
When a semiconductor chip which can be wire bonded must also be directly mounted on the insulating substrate in addition to electronic parts, a more complex problem is presented. When an electronic part is soldered on the conductor after the semiconductor chip is mounted on the conductor of the insulating substrate, the chip, if not well protected, is prone to contamination with solder flux or thermal influence, thereby degrading the characteristics of the integrated circuit. However, when the semiconductor chip is mounted on the conductor after the electronic part is soldered on the conductor of the insulating substrate, the chip mounting portion must be protected so as not to be contaminated with solder flux during soldering. Therefore, in either case, soldering cannot be easily performed by the dipping method unless a highly heat-resistant easy to remove protective film is used.
To resolve this problem, a method has been proposed which uses a cream solder wherein a solder powder is dispersed in a solvent together with a resin flux and which replaces the dipping soldering method. According to this method, the cream solder is applied at a predetermined position on the insulating substrate by screen printing or the like. After the electronic parts and the semiconductor chips are placed on the substrate, the structure is heated to harden the solder. However, in this method, since the cream solder re-flows upon being heated, the positioning precision of the electronic parts and chip is degraded.